Body armor is essential equipment for police and military. Currently, body armor is fielded only in specific high-risk scenarios, and is typically limited to chest and head protection. However, a significant percentage of battlefield injuries occur to the extremities, including arms, legs, hands, and neck. Armor for these extremities must offer protection from fragment and ballistic threats, without significantly limiting soldier mobility and dexterity.
Conventional body armor materials are typically comprised of many layers of polyaramid poly(phenylene diamine terephthalamide) fabric, sold by DuPont under the registerd name of KEVLAR®, with optional ceramic tile inserts. These materials are too bulky and stiff for application in extremities protection. A material is needed which can offer the equivalent ballistic performance of existing body armor materials, but with significantly more compactness and flexibility.
Shear thickening is a non-Newtonian flow behavior often observed in concentrated colloidal dispersions, and characterized by a large, sometimes discontinuous increase in viscosity with increasing shear stress (Lee and Reder, A. S. TAPPI Coating Conference Proceedings, p. 201, 1972; Hoffman, R. L., J. Colloid Interface Sci., Vol. 46, p. 491, 1974; Barnes, H. A., J. Rheol., Vol. 33, p. 329, 1989). It has been demonstrated that reversible shear thickening in concentrated colloidal suspensions is due to the formation of jamming clusters resulting from hydrodynamic lubrication forces between particles, often denoted by the term “hydroclusters” (Bossis and Brady, J. Chem. Phys., Vol. 91, p. 866, 1989; Foss and Brady, J. Fluid Mech., Vol. 407, p. 167, 2000; Catherall et al., J. Rheol., Vol. 44, p. 1, 2000). The mechanism of shear thickening has been studied extensively by rheo-optical experiments (D'Haene et al., J. Colloid Interface Sci., Vol. 156, p. 350, 1993; Bender and Wagner, J. Colloid Interface Sci., Vol. 172, p. 171, 1995), neutron scattering (Laun et al., J. Rheol., Vol. 36, p. 743, 1992; Bender and Wagner, J. Rheol., Vol. 40, p. 899, 1996; Newstein, et al., J. Chem. Phys., Vol. 111, p. 4827, 1999; Maranzano and Wagner, J. Rheol., Vol. 45, p. 1205, 2001a; Maranzano and Wagner, J. Chem. Phys., 2002) and stress-jump rheological measurements (Kaffashi et al., J. Colloid Interface Sci., Vol. 181, p. 22, 1997). The onset of shear thickening in steady shear can now be quantitatively predicted (Maranzano and Wagner, J. Rheol., Vol. 45, p. 1205, 2001a, and Maranzano and Wagner, J. Chem. Phys., Vol. 114, p. 10514, 2001) for colloidal suspensions of hard-spheres and electrostatically stabilized dispersions. This shear thickening phenomenon can damage processing equipment and induce dramatic changes in suspension microstructure, such as particle aggregation, which results in poor fluid and coating qualities. The highly nonlinear behavior can provide a self-limiting maximum rate of flow that can be exploited in the design of damping and control devices (Laun et al., J. Rheol., Vol. 35, p. 999, 1991; Helber et al., J. Sound and Vibration, Vol. 138, p. 47, 1990).
The general features of containment fibers for use in energy dissipating fabrics are high tenacity and high tensile modulus. These materials are also considered ballistic materials. At the same time, in many applications, it may be desirable to utilize a fabric having the benefits of relative low bulk and flexibility. To achieve such properties, polymeric fibers may be used. The fibers which may be preferred include aramid fibers, ultra-high molecular weight polyethylene fiber, ultra-high molecular weight polypropylene fiber, ultra-high molecular weight polyvinyl alcohol fiber and mixtures thereof. Typically, polymer fibers having high tensile strength and a high modulus are highly oriented, thereby resulting in very smooth fiber surfaces exhibiting a low coefficient of friction. Such fibers, when formed into a fabric network, exhibit poor energy transfer to neighboring fibers during an impact event. This lack of energy transfer may correlate to a reduced efficiency in dissipating the kinetic energy of a moving object thereby necessitating the use of more material to achieve full dissipation. The increase in material is typically achieved through the addition of more layers of material which has the negative consequence of adding to the bulk and weight of the overall fabric structure.
Among the most common uses for these so-called containment fabrics are in the use of body armor, and windings surrounding the periphery of turbine engines such as those found on commercial aircraft. Such an application is disclosed in U.S. Pat. No. 4,425,080 to Stanton et al. the teachings of which are incorporated herein by reference. The fabric is intended to aid in the containment of a projectile which may be thrown outwardly by rotating parts within the engine in the event of a catastrophic failure.
While the overall energy dissipating capacity of the fabric windings surrounding the engine is important, minimizing the thickness of the windings is also critical. Furthermore, economic considerations dictate that the number of fabric layers utilized for this purpose cannot be excessive. Thus, an effective containment structure should not require an excessive number of fabric layers to achieve the necessary levels of energy containment. It has been determined that the seemingly conflicting goals of improved kinetic energy containment and reduced material layers can, in fact, be achieved by improving the energy transfer between the adjacent fibers or yarns at the location of impact in the fabric network.
Several techniques are known for increasing the energy transfer properties between fibers or yarns but each of these known techniques has certain inherent deficiencies. One known method is to roughen the surface of the fibers or yarns by sanding or corona treatment. However, such roughening is believed to have limited utility due to the resultant degradation in the fiber.
Another method of increasing energy transfer between adjacent fibers or yarns is to coat the fabric with a polymer having a high coefficient of friction. One deficiency in this practice is the formation of fiber-to-fiber bonds. Such bonding may result in stress reflections at yarn crossovers during impact by a moving article, which cannot be transferred away from the impact region. Another deficiency is the large weight gain typical of coatings, which may be ten percent or more. A further limitation of this approach is a significant decrease in fabric flexibility due to the addition of the relatively stiff polymer coating. A related method is to use a sticky resin that creates adhesion between the fibers, as disclosed in U.S. Pat. No. 1,213,118 to Lynch, but this technique has the same inherent deficiencies of fiber to fiber bonding and increased weight as exhibited by coatings.
Yet another method for improving the energy transfer between fibers or yarns in a containment fabric is core spinning of high strength fibers in combination with weaker fibers having a higher coefficient of friction as disclosed in U.S. Pat. No. 5,035,111 to Hogenboom. However, these relatively high friction fibers may reduce the overall fabric strength.
Dischler et al. (U.S. Pat. No. 5,776,839) used KEVLAR® fibers coated with a dry powder that exhibits dilatant properties. Dilatant properties refer to increases in both volume and viscosity under flow. In their work, the fibers demonstrated an improved abililty to distribute energy during ballistic impact due to the enhanced inter-fiber friction.
Schuster et al (U.S. Pat. No. 5,854,143) also describe the use of dry dilatant agents in a fabric carrier to improve ballistic protection. In their approach, the dilatant agent is a polymeric powder which is applied to the fabric while suspended in a carrier fluid, and subsequently dried to leave behind the dilatant solid.
Gates (U.S. Pat. No. 3,649,426) describes the use of a dilatant dispersion consisting of small rigid particulates suspended in an environmentally stable liquid, such as glycerin. In this case, the solid-liquid dispersion is dilatant, and remains flowable in the armor material. The dilatant dispersion is confined in flexible cellular compartments which could be placed behind conventional protective armors. However, the necessity of a cellular containment structure results in a material system which is bulky, heavy, and relatively inflexible.